1. Field of the Invention
The present invention relates to an image forming apparatus such as copiers, printer, facsimiles, and digital multifunction peripherals, and a gradation correction method for the image forming apparatus.
2. Description of Related Art
In general, in an electrophotographic image forming apparatus, a charged photoconductor is so irradiated with laser light based on image data that an electrostatic latent image is formed, and the latent image is developed with a developer containing toner to visualize the latent image, forming a toner image. The toner image is directly or indirectly transferred to a sheet, and a fixing device then heats and pressurizes the sheet to fix the toner image thereto. In this way an image is formed on the sheet.
In such an image forming apparatus, the image density of the toner image varies due for instance to degradation of the photoconductor, the developer, and other image formation-related components over time and to changes in temperature, humidity, and other environmental factors around the apparatus, disadvantageously resulting low quality of an output image formed on the sheet. More specifically, a phenomenon occurs in which the gradation levels of an input image are not reproduced in an output image with high fidelity. In view of the foregoing drawback, in a conventional image forming apparatus, image stabilization control for stabilizing the density of an image, such as gradation correction, is performed so that the gradation levels and other parameters of an input image are stably reproduced in an output image (see, e.g., Japanese Patent Application Laid-Open No. 2006-259261).
In an image forming apparatus, for example, a gradation pattern image (also referred to as a “patch band”) formed of a plurality of gradation levels is formed as a test toner image on the photoconductor, and gradation correction is performed based on the densities of the gradation pattern image. This gradation correction is performed to correct image gradation characteristics when turning on power, when returning from sleep, when the print count has reached a predetermined number, or when the external environment has significantly changed.
An example of the image stabilization control is gradation correction (gamma correction) performed by forming CMYK color toner patterns on an intermediate transfer belt, which is an image bearing member, detecting the densities of the toner patterns with an optical sensor, generating gradation correction data based on the detection results (gradation characteristics), and feeding the gradation correction data back to image formation conditions, such as charging potential, developing potential, and the amount of light exposure.
Specifically, when the image forming apparatus performs the gradation correction, a gradation pattern image is formed on the intermediate transfer belt, and the density of each gradation of the gradation pattern image is detected with a density detection sensor. A desired image density is achieved by adjusting a development bias in such a way that the detected densities agree with target densities set in advance (that is, by performing gradation correction). In practice, the gradation correction is performed by correcting input gradation levels of image data based on the densities detected by the density detection sensor in such a way that each gradation agrees with the predetermined density in a gradation conversion table in a control section.
Japanese Patent Application Laid-Open No. 2006-259261 discloses an image forming apparatus that forms a gradation pattern image of each color for gradation correction on an image bearing member based on image data having predetermined gradation levels, measures the densities of the formed color gradation pattern images with a reflective density detection sensor, creates a gamma correction curve based on the measurement results, and regularly updates the gamma correction curve.
In the image stabilization control described above, it is necessary to accurately detect the densities of the gradation pattern images formed on the image bearing member and accurately figure out the current gradation characteristics. However, what is called image unevenness can affect a plurality of gradation patches that form each of the gradation pattern images formed on the image bearing member. The image unevenness used herein is a phenomenon in which fatigue and other types of degradation of a development roller, the photoconductor, a transfer belt, or any other member that forms an image formation section cause the density of an output image output on a sheet to decrease or increase in the moving direction (also referred to as “sub-scanning direction”) of the image bearing member. In this case, the image unevenness affects detected densities of the plurality of gradation patches formed on the sheet in the moving direction of the image bearing member. In particular, detected densities of high-density gradation patches are not accurate. The gradation characteristic of the image formation section that has been calculated from the detected densities of the plurality of gradation patches is therefore not accurate, and gradation correction performed based on the resultant gradation characteristic will not provide appropriate image stabilization control.
Further, what is called reflection unevenness may affect the detection of the plurality of gradation patches that form each of the gradation pattern images formed on the image bearing member. The reflection unevenness used herein is a phenomenon in which variation in projections and depressions that occurs in a base surface of the image bearing member along the moving direction of the image bearing member causes the reflectance of the base surface to vary when the base surface is irradiated with laser light for forming an electrostatic latent image. In this case, the reflection unevenness affects detected densities of the plurality of gradation patches formed on a sheet in the moving direction of the image bearing member. In particular, detected densities of low-density gradation patches are not accurate. The gradation characteristic of the image formation section that has been calculated from the detected densities of the plurality of gradation patches is therefore not accurate, and gradation correction performed based on the resultant gradation characteristic will not provide appropriate image stabilization control.
Poor accuracy in the detection of the gradation pattern images lowers the accuracy in the gradation correction accordingly, and a large number of ideas have therefore been put into practice to detect the densities of the gradation pattern images with high accuracy.
Japanese Patent Application Laid-Open Nos. 2010-134366, 2008-26551 and 2006-343679 disclose technology that uses a plurality of gradation pattern images. They also disclose technology in which in order to cancel density unevenness that occurs at the cycle of a developing roller, an intermediate transfer belt, or a photoconductor (i.e., cyclic density unevenness), a plurality of gradation patterns are positioned at an interval of ½ or M/N (where M and N are coprime natural numbers) of the above-described cycles.
Japanese Patent Application Laid-Open Nos. 2010-171689 and 2011-64715 disclose technology for limiting a decrease in accuracy in gradation correction due to density unevenness or lack of density by skillfully arranging gradation pattern images.
In these technologies, to shorten the overall period required to perform gradation correction by shortening the period required to detect the densities of a gradation pattern image, the length of the gradation pattern image may be shortened. Shortening each of the gradation patches that form the gradation pattern image, however, increases the possibility of wrong detection of the densities of the gradation patches, resulting in a decrease in accuracy of gradation correction.
Forming a plurality of gradation pattern images as disclosed in Japanese Patent Application Laid-Open Nos. 2010-134366, 2008-26551, 2006-343679, 2010-171689 and 2011-64715 described above can relatively increase the total length of the gradation patches, which lowers the possibility of wrong detection of the densities of the gradation patches. Depending on the arrangement of the gradation pattern images, however, the period required to detect the densities of the gradation pattern images increases.
FIGS. 1 and 2 show examples of the arrangement of a plurality of gradation pattern images 3-1 and 3-2 formed on intermediate transfer belt 1.
FIG. 1 shows a case where the same gradation pattern images 3-1 and 3-2 are disposed in series with each other along the moving direction of intermediate transfer belt 1. In the case shown in FIG. 1, when intermediate transfer belt 1 moves in the direction indicated by the arrow, density detection sensor 2 fixed to an image forming apparatus measures the densities of gradation pattern images 3-1 and 3-2 from left to right.
FIG. 2 shows an example where the same gradation pattern images 3-1 and 3-2 are disposed in parallel to each other along the moving direction of intermediate transfer belt 1. In the case shown in FIG. 2, when intermediate transfer belt 1 moves in the direction indicated by the arrow, density detection sensors 2-1 and 2-2 fixed to an image forming apparatus measure the densities of gradation pattern images 3-1 and 3-2 from left to right.
In both cases shown in FIGS. 1 and 2, the densities of the gradation patches in the same gradation pattern images 3-1 and 3-2 are averaged, and gradation correction is performed based on the averaged densities. This provides reliable measured densities that are not greatly affected, for example, by density unevenness.
In the arrangement shown in FIG. 1, densities of gradation pattern images 3-1 and 3-2 disposed in series with each other are detected by single density detection sensor 2. This arrangement disadvantageously requires a density detection period longer than the case where densities of gradation patterns 3-1 and 3-2 disposed in parallel to each other are detected by two density detection sensors 2-1 and 2-2 as shown in FIG. 2. In a simple calculation, the arrangement shown in FIG. 1 requires a density detection period twice longer than the arrangement shown in FIG. 2.
In these arrangements, it is assumed that there is scratch 4 that runs along the width direction (also referred to as “main scanning direction”) of intermediate transfer belt 1. In this case, the arrangement shown in FIG. 1 provides more correct detected densities than the arrangement shown in FIG. 2. That is, in the arrangement shown in FIG. 1, even when scratch 4 is present in one of the gradation patches that form gradation pattern image 3-1, a detected density of the gradation patch with scratch 4 can be replaced with a detected density of the gradation patch having the same gradation in gradation pattern image 3-2. In contrast, in the arrangement shown in FIG. 2, since scratch 4 is present both in gradation patches having the same gradation level in gradation pattern images 3-1 and 3-2, it is difficult to provide correct detected densities of the gradation patches where scratch 4 is present. That is, in the arrangement shown in FIG. 2, which can lower the degree of an adverse effect of a local scratch, it is difficult to eliminate an adverse effect of scratch 4 or unevenness present across the width of intermediate transfer belt 1.
In addition to scratch 4, intermediate transfer belt 1 has other density affecting factors present along the width of intermediate transfer belt 1, such as curling of the intermediate belt, streaks derived from the photoconductive drum.
Conventionally, sufficient studies have not been conducted on technologies for not only shortening the density detection period but also limiting a decrease in density detection accuracy due to density affecting factors in the direction perpendicular to the direction of arranging gradation pattern images (scratch 4 extending along the width of intermediate transfer belt 1 in FIGS. 1 and 2).